Can a hotel charge a cancellation fee if they changed the rates that we signed and agreed to - now too?

As we were checking ut last Sunday, we made reservations at the same hotel in Lake George NY for bike week 2012. I signed a confirmation sheet for a total of $750 for 4 nights. Yesterday in the mail, we got another confirmation sheet from them - jacking the rates up by $206 for the 4 nights. We did not sign or agree to those increased rates - we cannot afford that and want to cancel - the hotel still wants to charge us a $100 cancellation fee? Can they do that - they changed the contract without our knowledge or consent?